Project Summary/Abstract Targeted cancer therapy requires complete and sustained pharmacological inhibition of the respective target, which is often challenging to achieve due to the lack of amenable sites in non-enzyme cancer targets such as transcription factors. The clinical success of thalidomide analogs demonstrates that small molecules that promote targeted protein degradation can overcome this limitation, providing a new pharmacologic toolbox. Most small molecule degraders induce new protein?protein interaction through a ?molecular glue? mechanism that bridges a ubiquitin E3 ligase and the target protein. Over 600 ubiquitin E3 ligases exist in the human proteome, allowing many points of intervention to tune their substrate specificity to degrade neo-substrates, However, directly re-wiring ubiquitin ligase to degrade target protein is still challenging; only few E3 ligases such as CRBN, VHL, and DCAF15 have been repurposed for targeted protein degradation. In order to devise general strategies for degrader discovery, it is important to acquire comprehensive mechanistic understanding of how chemical ligands facilitate the degradation of neo-substrates. A recently described small molecule, BI-3802 induces rapid ubiquitination and degradation of BCL6, an oncogenic transcription factor that drives diffuse large B cell lymphoma (DLBCL). BCL6 degradation by BI-3802 treatment results in superior anti-proliferative effects on DLBCL cells, but the mechanism and molecular machinery that are involved remain elusive. In the F99-phase of this proposed research, Hojong Yoon will use the combination of functional genomics and structural biology approaches to dissect the molecular mechanism of BI-3802-induced BCL6 degradation. In the K00-phase of this proposed research, Hojong Yoon will employ his new finding of chemically-inducible polymerization from the F99-phase to develop a novel method of pharmacological intervention. The proposed work will provide a better understanding of small molecule-induced protein degradation to enable the optimization of current small molecule degraders and the development of degraders for new targets. Ultimately, the new insights from this research will lead the development of innovative pharmacological modalities that can be tailored and applied to currently intractable cancers.